Solid-State Phenomena, 82-84 , 565-567, 2002. Defect related current instabilities in InAs/GaAs and AlGaAs/GaAs structures? Zs. J. Horváth 1 , S. Franchi 2 , A.Bosacchi 2 , P. Frigeri 2 , E. Gombia 2 , R. Mosca 2 , and Vo Van Tuyen 1 1 Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science Budapest 114, P. O. Box 49, H-1525 Hungary 2 MASPEC Institute - C.N.R. Parco Area delle Scienze 37a, 43010 Fontanini-Parma, Italy, Keywords : GaAs/InAs, GaAs/AlGaAs, quantum dots, current instability, defects, minority injection, recombination. Abstract. Unexpected excess current was obtained in GaAs/InAs quantum dot structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. It has been concluded that the excess current is a minority injection current connected with recombination through defects originated from the formation of QDs. The instabilities are connected with unstable occupation of energy levels induced by the above defects, which depend on temperature and on the current level. Excess currents have also been obtained in annealed AlGaAs/GaAs structures. These excess currents exhibited memory effect, which was probably connected with formatin of defects during annealing. Introduction Recently the electrical characteristics of InAs quantum dot (QD) and quantum well (QW) structures embedded in GaAs confining layers were studied [1,2]. Unexpected excess current was obtained in the QD structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. Excess currents exhibiting memory effect, were also obtained in annealed AlGaAs/GaAs structures. In this paper these results are presented and discussed. Experimental The InAs QD and QW structures embedded in GaAs confining layers were grown by MBE and Atomic Layer MBE (ALMBE); as the coverages of the highly mismatched InAs layers increase beyond a critical value Q tr (1.6 ML for InAs on GaAs) the growth of continuous two-dimensional InAs layers evolves in the formation of self-aggregated three- dimensional QDs, according to the Stranski-Krastanov growth mode. The structures consist: (i) of GaAs buffer layers with electron concentration of 2x10 16 cm -3 and thickness of 1 μm (ii) of QDs or QWs obtained using InAs coverages of 3.0 ML and 1.0 ML, respectively, and (iii) of GaAs upper confining layers, with electron concentration of 2x10 16 cm -3 . The buffer layers were grown by MBE at 580 °C; then the growth was interrupted for 210 s to lower the growth temperature to that used for the deposition of InAs QWs or QDsby ALMBE (at 460 °C). The upper confining layers were grown after interruptions of 210 s by ALMBE (at 400 °C) for 0.4 micrometers and by MBE (at 580 °C) for the subsequent 0.6 μm. This growth procedure was determined so as to optimise the photoluminescence properties of the InAs/GaAs QDs. As 4 /Ga and As 4 /In beam equivalent pressure ratios of ~16 and ~ 28 were used; the In and Ga fluxes were adjusted for InAs and GaAs growth rates of 0.16 ML/s and 1.0 ML/s, respectively; during ALMBE, the growth rate was set at 0.2 ML/cycle and the As supply time was chosen so as to give a sharp (2x4) surface reconstruction at the end of the As cycle. The substrates were radiatively heated at growth temperatures Tg, which was measured by a suitable optical pyrometer for Tg of 450 °C and by a thermocouple (TC) not in direct contact with the substrate for Tg < 450 °C. For Tg < 450 °C, the TC readings were corrected by the difference (~200 °C) between the TC and the optical pyrometer values measured at Tg > 450 °C. The InAs coverages were determined by using the growth rate measured by observing the 2D- 3D growth transition on a calibration substrate just before the preparation of the QDs or of the QWs, and by assuming that the transition takes place at the generally accepted value Θ tr of 1.6 ML. This value is very close to that (1.57 ML)